How may I achieve the effect of insetting a concave n-gon?
An alternative and perhaps more accurate method would be to use the Solidify modifier twice.

Trace out the outside walls outer edge ground plan then add a Solidify modifier to create the inner edges of these walls. In the example below I set my walls to have 0.2m thickness and selected only the Even Thickness option. The solidify modifier needs faces to work with so the outer walls were extruded up 1 meter.

After applying this first modifier add a second. This will be for the insulation thickness. In object mode apply this modifier.
 The second modifier created unwanted faces ouside of the outer walls so these will need to be deleted. Also delete the extruded edges to return to a simple 2D ground plan.



With Snapping to vertex enabled use extrude to add the missing inner wall and insulation.